Technology: Sculpted fibre gathers up laser's lost light

When light is shone from a laser into the end of an optical fibre, it
spreads out so much that half of it is usually lost. Now researchers in
the US have developed a way to sculpt the end of the fibre into a precision
lens which can gather 90 per cent of the laser’s light into the fibre.

The light-carrying core of a fibre used in communications is only about
0.01 millimetres across. This is slightly larger than the light-emitting
area at the end of semiconductor lasers which are used to beam light into
the fibre. But the beams from the lasers typically diverge at angles of
up to 45 degrees, so only 5 to 10 per cent of the light will stay in the
core. What is needed is a lens to focus the light into parallel rays.

Researchers first made the tips of fibres into lenses simply by melting
the ends. The surface tension of the molten glass pulls the melted tip into
a hemisphere. But melting the whole tip did not focus light strongly enough,
so stronger lenses were developed by etching the cladding from the end of
the fibre and melting only the core. This traps half the light in the core
and is a commonly used technique for optical fibre communications.

But Herman Presby of AT&T Bell Laboratories in New Jersey says that
an ideal lens would have a hyperbolic rather than hemispherical surface.
He produces this shape by focusing pulses of light from a 25-watt carbon
dioxide laser onto the end of the fibre as it is spun around its axis. The
computer-controlled laser removes glass from the outer parts of the fibre
and heats the tip, producing a smooth-surfaced hyperbolic lens which focuses
light into the core with 90 per cent efficiency.

Although the process sounds complex, Presby says it takes less than
30 seconds, and is simpler than etching off the cladding and melting the
fibre tip. AT&T has patented the process, and the company is investigating
its use in production.